ata: libata-scsi: do not use the deferred QC feature for ATA_DEFER_PORT

The deferred QC feature was meant to handle mixed NCQ and non-NCQ commands,
i.e. for return value ATA_DEFER_LINK.

ATA_DEFER_PORT is returned by PATA drivers, but also certain SATA drivers
like sata_mv and sata_sil24 that uses ap->excl_link to workaround hardware
bugs in these HBAs. Regardless of the reason, using the deferred QC feature
for ATA_DEFER_PORT is always wrong, and will break the ap->excl_link usage
of the SATA drivers that rely on that feature.

Modify ata_scsi_qc_issue() to only use the deferred QC feature when mixing
NCQ and non-NCQ commands, i.e. ATA_DEFER_LINK.

Fixes: 0ea8408 ("ata: libata-scsi: avoid Non-NCQ command starvation")
Tested-by: Tommy Kelly <linux@tkel.ly>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>

Author: floatious

drivers/ata/libata-scsi.c

@@ -1789,11 +1789,11 @@ static int ata_scsi_qc_issue(struct ata_port *ap, struct ata_queued_cmd *qc)
 		goto defer_qc;
 	case ATA_DEFER_PORT:
 		ret = SCSI_MLQUEUE_HOST_BUSY;
-		goto defer_qc;
+		goto free_qc;
 	default:
 		WARN_ON_ONCE(1);
 		ret = SCSI_MLQUEUE_HOST_BUSY;
-		goto defer_qc;
+		goto free_qc;
 	}
 
 issue_qc:
@@ -1813,6 +1813,7 @@ static int ata_scsi_qc_issue(struct ata_port *ap, struct ata_queued_cmd *qc)
 		return 0;
 	}
 
+free_qc:
 	/* Force a requeue of the command to defer its execution. */
 	ata_qc_free(qc);